The present invention relates to an apparatus for measuring a Q value which is a parameter for the evaluation of the transmission characteristic of an optical communication system.
In the evaluation of the characteristic of an optical fiber communication system, a bit error rate (BER) has usually been used as a parameter for the performance evaluation. The bit error rate is defined by the rate of the number of erroneously received bits to the total number of bits received per second; when the bit error rate is very low (for example, 10.sup.-14 or below), its measurement is time-consuming. In a system having a transmission rate of five gigabits per second, for instance, a minimum of six hours or so is needed to measure the bit error rate of 10.sup.-14 or below.
A system employing an optical amplifier as an optical repeater is now approaching practical use as a next-generation optical communication system; it has been reported, however, that such a system suffers a fading phenomenon that the bit error rate undergoes temporal variations owing to characteristics of optical components of the system (S. Yamamoto et al., "OBSERVATION OF BER DEGRADATION DUE TO FADING IN LONG-DISTANCE OPTICAL AMPLIFIER SYSTEM", IEEE Electronics Letters, vol. 29, no. 2, pp. 209-210). If we need many hours to measure the bit error rate in such a system, the measured bit error rate will be limited under the influence of the fading phenomenon. In a six-hour measurement of a system which primarily demonstrates under 10-14 bit error rate, for instance, if the bit error rate becomes worse than 10.sup.-10 even at an instant, the mean value of the bit error rate for six hours will not reach 10.sup.-14, making it impossible to measure the system performance. Hence, in such a system, the bit error rate counts for nothing as the parameter for the evaluation of the characteristic of the system.
It is a Q value that has been proposed as the parameter for the evaluation of such a system as mentioned above. The Q value indicates the SN ratio of an electric signal in an optical receiver and is defined by Eq. (1). EQU Q=.vertline..mu..sub.m -.mu..sub.s /(.sigma..sub.m +.sigma..sub.s) (1)
where .mu.m is the mean value of mark levels, .mu..sub.s the mean value of space levels, .sigma..sub.m a standard deviation of the mark level and is a standard deviation of the space level. The value BER (V.sub.th) of the bit error rate at a certain signal decision threshold level V.sub.th is expressed by Eq. (2). EQU BER(V.sub.th)=[erfc{(.vertline..mu..sub.m -V.sub.th .vertline.)/.sigma..sub.m }+erfc{(.vertline..mu..sub.s -V.sub.th .vertline.)/.sigma..sub.s }]/2 (2)
where erfc is indicative of an error function. At the signal decision threshold level which makes the value EBR optimal, Eq. (2) is rewritten using Eq. (1) as follows: EQU BER=erfc(Q) (3)
It is seen from Eq. (3) that the measurement of the Q value and the measurement of the bit error rate are equivalent, since the Q value has a one-to-one correspondence with the bit error rate. The measurement of a very low bit error rate requires as long time as several hours, but the measurement of the Q value can be made in a relatively short time ranging from several to tens of minutes, and hence is not so badly affected by the fading phenomenon as in the case of the bit error rate measurement.
One possible method for measuring the Q value is to use a digital storage scope and calculate the Q value from histograms of mark levels and space levels stored or accumulated with time (P. K. Runge et al., "9000 KM OPTICAL AMPLIFIER SYSTEM TEST FACILITY RESULTS", Suboptics '93, S4.1). The latest digital storage scope incorporates histogram mean value and standard deviation calculating features and allows ease in measuring the Q value. However, the Q value obtainable with this method does not exactly correspond to the bit error rate owing to a quantization error of the digital storage scope.
To measure the Q value which corresponds precisely with the bit error rate, there has been proposed a method which measures the bit error rate at a plurality of decision threshold levels while changing the decision threshold of an error detector and calculates the Q value by interpolating the bit error rate thus measured (N. S. Bergano et al., "Margin Measurements in Optical Amplifier Systems", IEEE Photonics Technology Letters, vol. 5, no. 3, pp. 304-306).
Since the error detector available at the present has only one signal decision circuit, however, the abovementioned method inevitably involves the repetition of works of changing the signal decision threshold level of the error detector and measuring the bit error rate at each decision threshold level. As a result, the measurement takes several to tens of minutes, leading to a defect that the measured values become less accurate under the influence of the fading phenomenon.